1. Field of the Invention
The present invention relates to a multi-layer type electric double-layer capacitor, a type of energy storage system. More precisely, the invention relates to a multi-layer type electric double-layer capacitor that comprises composite electrodes, in which each composite electrode is formed by firmly bonding a carbon electrode to an electroconductive polymer sheet with no adhesive therebetween to integrate them.
Power leveling devices that are for storing the excess power in the nighttime so as to reduce the peak power consumption in the daytime, and economical hybrid cars that release clean exhaust gas are being much developed these days for energy saving and for global environment protection. For these, it is urgently necessary to develop high-voltage storage devices such as electric double-layer capacitors. In the field of these applications, desired are high-voltage storage devices, one of which is a multi-layer type energy storage device that comprises unit cells connected in series, and this is the most hopeful in the art.
The electrode for electric double-layer capacitors comprises a polarizable electrode layer formed on a collector, in which the polarizable electrode layer is essentially formed of active carbon having a large specific surface area. Electric double-layer capacitors are so designed that the unit cells therein each composed of a pair of such electrode layers with an insulating separator put therebetween are impregnated with an electrolytic bath. Applying a voltage between the two electrodes therein, the capacitor of this type stores the resulting charges in the electric double-layer in the interface between the electrode and the electrolytic bath, and therefore, this can store a far larger quantity of electricity than conventional capacitors. When compared with secondary batteries that undergo redox reaction, the quantity of electric energy which such an electric double-layer capacitor can store therein is not so large. However, the power output capability of the capacitor of that type, that is, the ability thereof to store and release a large quantity of electric power within a short period of time is about 10 times that of a conventional secondary battery, and, in addition, the charge-discharge cycle life of the capacitor of that type is at least 100,000 cycles, that is, the life thereof is at least 100 times that of a conventional secondary battery. Having these advantages, electric double-layer capacitors are now put into practical use in various industrial fields, for example, for back-up power sources for IC and LSI memories and actuators.
2. Description of the Related Art
The electromotive force of such an electric double-layer capacitor generally falls between 1and 3V or so, though depending on the composition of the electrolytic bath. In case where the capacitor of the type is used for a high-voltage power source, unit cells that are prepared separately are connected in series or are stacked to ensure the intended high voltage. In particular, multi-layer type electric double-layer capacitors are preferred since their space efficiency is high. Accordingly, the characteristics of multi-layer type electric double-layer capacitors are significantly influenced not only by the unit cells constituting them but also by the cell-to-cell contact resistance of the multi-layer type cells. For reducing the internal electric resistance of the multi-layer type cells, needed is an intercollector of high electroconductivity; and for increasing the energy density and the output density thereof, needed is a thin intercollector of low specific gravity. For these, proposed is a bipolar interelectrode formed by bonding a carbon electrode to both surfaces of an intercollector.
The collector, one constituent component of an electric double-layer capacitor is required to have a reduced internal electric resistance so as to be able to supply a large electric current, for which, for example, used are electroconductive foil, titanium plates and electroconductive polymer sheets. In case where a carbon electrode is bonded to a metallic collector, the two are mechanically bonded under pressure, or are bonded with an adhesive put between them. However, the former method of mechanically bonding them is problematic in that it often produces vibration or temperature fluctuation to cause cell volume change that may lead to the increase in the contact resistance of the bonded two, the reduction of the power performance thereof and the increase in the ohmic loss thereof. On the other hand, the latter method of bonding the two with an adhesive put therebetween is defective in that the adhesive used deteriorates since it is all the time kept in contact with the organic electrolytic bath and the acidic or basic aqueous electrolytic bath, and, as a result, the cell lacks long-term stability. For these reasons, it is desired to integrate the collector and the electrode with no adhesive put therebetween.
In JP-A 250380/1996, described is a method of dissolving and mixing active carbon and a binder polymer in an organic solvent such as toluene, then applying the resulting mixture onto a collector electrode and drying it thereon. However, this method has a lot of problems to solve for solvent recovery, environmental protection, human health, cost reduction and the like.
In JP-A 179711/1987 and 16506/1987, described is a method of emulsifying active carbon powder and a binder polymer followed by applying the resulting emulsion onto a collector electrode in the same manner as above. In this method, used is an aqueous emulsion which is favorable for environmental protection and human health. However, as the solid content thereof is low, the emulsion used in the method is problematic in that its production efficiency is not good and, in addition, it forms only a thin film and the mechanical strength of such a thin film made of it is low. Moreover, the technique of integrating the active carbon electrode with the collector electrode in the method requires an adhesive or an electroconductive adhesive, which is therefore problematic in point of the troublesome adhesive-coating step, the adhesion strength of the two bonded with such an adhesive, and the time-dependent change of the properties of the bonded structure including the chemical resistance thereof.
In consideration of the prior-art problems stated above, the present invention is to construct a bipolar interelectrode of low internal electric resistance by bonding a carbon electrode to a collector with no adhesive put therebetween and to provide a multi-layer type electric double-layer capacitor constructed by placing in layers unit cells each containing that bipolar interelectrode.
The present inventors have found that a carbon electrode can be firmly bonded by heat-sealing to an electroconductive sheet with no adhesive, and that the electroconductive polymer composite electrode thus constructed is significantly effective for solving the problems mentioned above. On the basis of these findings, we have completed the present invention.
Specifically, the multi-layer type electric double-layer capacitor which the invention provides herein comprises;
a plurality of combinations of an intermediate electroconductive polymer composite electrode and a separator placed in layers between a pair of terminal electroconductive polymer composite electrodes, wherein;
the intermediate electroconductive polymer composite electrode is composed of an electroconductive sheet made of a polymer substance to which electroconductivity is imparted and which has an apparent melt viscosity of from 300 to 3,000,000 poises, and a carbon electrode heat-sealed to at least one surface of the sheet, and
the carbon electrode and the separator contain an electrolytic bath infiltrated thereinto, that is, the carbon electrode and the separator are impregnated with an electrolytic bath.